There are a variety of known inspection systems which illuminate a flat surface to be inspected such as a laser-etched art work semiconductor lead framework. However, most of the known inspection systems are of a relatively large size which do not easily fit within the small confines of currently available semiconductor processing equipment.
In particular, known arrangements typically affix a ring of LEDs to the underside of rather large and bulky inspection equipment. The ring of LEDs is centered about the optical axis which extends normal to the inspection surface. This illumination geometry is useful for imaging "mirror melting" by a laser of desired art work on a diffusely reflecting metal surface. In the area affected by the laser, the diffused surface finish is melted to convert that surface area into a highly specular surface finish. This specular surface finish reflects the low-angle dark field illumination off at an equivalent low-angle causing it to appear dark in the field of view. The diffuse background finish reflects some of the incident low-angle illumination along the optical axis into the camera lens and hence that area appears bright. This combination causes the "mirror melting" laser mark to appear in high contrast, e.g. black on a white surface, rendering it fairly easy to decipher by conventional machine vision systems.
If the lead frame surface finish is highly specular, however, the dark field illumination geometry described above will cause the surface to appear black, hence rendering invisible any "mirror melting" art work, e.g. a dark field on a dark field. On such a highly specular surface for laser etching to be visible under a dark field illumination the surface must be optically "roughed up" by the laser. For example, the surface must be etched so as to form small craters or pits. Under dark field illumination, only the rim of the laser pits will reflect light to the camera while the valleys of the pits will reflect the light to the surrounding environment. If the pits are small enough and spaced closely enough together they can be made to appear as a "solid" feature. If the pits are isolated and enlarged, however, they appear as bright rings on a dark background, potentially causing problems with the inspection algorithms currently used in prior art systems.